Casein which is a main component of a milk protein exhibits excellent emulsifying and thickening activities in a system within a neutral pH range or a system containing no salt or containing a salt in a low concentration and, therefore, it is widely employed as a raw material or an additive for food of such systems. However, casein aggregates and precipitates at a pH range of not more than about 4.6 which is the isoelectric point thereof. Further, casein also aggregates in the presence of a large amount of a salt such as sodium chloride, etc. and salting-out of casein occurs. Accordingly, in a system within an acidic pH range or containing a large amount of a salt, excellent properties and functions of casein are spoiled and it is very difficult to use casein in food of an acidic pH range or food containing a large amount of a salt.
Under these circumstances, the present inventors have intensively studied in order to obtain a processed casein which can exhibit sufficient emulsifying and thickening activities without salting-out even in the presence of a large amount of a salt, and excellent emulsifying activity without aggregation even in an acidic system. As the result, the present inventors have found that, by subjecting casein and an acidic polysaccharide to a heat treatment under certain conditions, it is possible to obtain casein which is resistant to both acids and bases and can exhibit excellent properties and functions thereof even in food of an acidic pH range or food containing a large amount of a salt as well as in food within a neutral pH range or food containing no salt or a small amount of salt.